dalex's blog

All you should know about comparators in C++
By dalex, 17 months ago, translation, In English

Please put to main page. So we won't see blogs like https://codeforces.com/blog/entry/70236 in the nearest future.

In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.
In C++, comparator should return false if its arguments are equal.

Hope you remember it and will never make such a stupid mistake!

 
 
 
 
  • Vote: I like it
  • +304
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Comments Comments (23)
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17 months ago, # |
  Vote: I like it -16 Vote: I do not like it

I didn't know about this before xD

What happens if you return true instead? Runtime error always? Probably not.

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17 months ago, # |
← Rev. 2   Vote: I like it +34 Vote: I do not like it
  1. https://codeforces.com/blog/entry/70358?#comment-548342 by nishant403
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14 months ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

It's sad that you only see this kind of posts after you get wrong answer.
Why isn't this mentioned in tutorials, the only way to know about this is to read the documentation.(or blogs like this which you can never find without searching for)

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    14 months ago, # ^ |
      Vote: I like it +31 Vote: I do not like it

    You are supposed to read documentation for the function BEFORE using this function

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14 months ago, # |
← Rev. 4   Vote: I like it +44 Vote: I do not like it

That's not all you should know about it.

In fact, it should be a Strict Weak Ordering.(transitivity and transitivity of equivalence are also needed).

There's a well-known problem which can be solved by Johnson's rule, and a simple greedy solution which sorts the jobs lacks of transitivity of equivalence. The wrong comparator is $$$P(i, j)=\min(a_i,b_j)<\min(a_j,b_i)$$$ , it should be $$$P(i, j)=\begin{cases}a_i< a_j&\min(a_i,b_j)=\min(a_j,b_i)\\min(a_i,b_j)<\min(a_j,b_i)&\text{otherwise}\end{cases}$$$ .

I don't know any English material about this problem except Wikipedia, so here is this problem in Chinese. In this article (also in Chinese) written by me, I discussed about it.

If anyone is interested in it, I can write a blog about it (in English) :)

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    14 months ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    I will be grateful if you write the blog in english, thanks.

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      14 months ago, # ^ |
      ← Rev. 2   Vote: I like it 0 Vote: I do not like it

      See this.

      I didn't translate that Chinese blog. I wrote a new one instead :)

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        14 months ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        Thanks..

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        6 weeks ago, # ^ |
          Vote: I like it -14 Vote: I do not like it

        Hey, thank you so much for writing this amazing blog! Searching on Google brought me here :)

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14 months ago, # |
  Vote: I like it 0 Vote: I do not like it

I guess most learn this the hard way, I certainly did.

So now, I always give my comparator function the name "lessThan". Instead of something like "cmp" before.

Then you always remember to handle equals correctly.

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